The periodic table puts the elements in order of atomic number. Elements in one group have the same number of valence electrons, so they behave alike. This note covers the history in brief, then the trends and the period 3 chemistry that the book asks about.
Think of a school with 118 students and no class lists. You could not tell who sits with whom. The periodic table is the class list. Each column is a group of friends, and each row is a year.
This note follows our lessons History of the Periodic Table, The Modern Periodic Table and Periodic Trends. They follow the new PECTAA Chemistry 11 book (2025-26), Chapter 1. The book numbers the groups 1 to 18, and these notes use that numbering.
What to know: history
- Döbereiner, 1829. He grouped elements in triads with similar properties. The middle atomic weight is roughly the average of the other two. Example: lithium, sodium and potassium (7, 23 and 39).
- Newlands, 1864. He arranged the 62 known elements by increasing atomic mass. Every eighth element resembled the first.
- Mendeleev, 1869. He arranged 63 elements by increasing atomic mass, with similar elements in groups. His success came from leaving gaps for undiscovered elements and predicting their atomic mass and properties. The predictions proved accurate when the elements were found.
- Moseley, 1913. He found the exact atomic numbers using X-ray emission. This removed flaws in Mendeleev's table. It changed the law to atomic number.
Modern Periodic Law: the physical and chemical properties of elements are periodic functions of their atomic numbers.
Key points: the modern table
- 118 elements are in ascending order of atomic number.
- There are 7 periods (rows) and 18 groups (columns).
- Elements in a group have similar chemical properties because they have the same number of valence electrons. Physical properties change gradually down the group.
- Metals tend to lose electrons and form positive ions. Non-metals tend to gain electrons and form negative ions. Metalloids separate the two and show some properties of both. The stair-step line starts at boron and ends at polonium. It includes Si, Ge, As, Sb and Te.
Period and group tell you the configuration.
- The period number is the number of shells.
- The group number gives the valence electrons. The book's example: period 3 and group 13. It has 3 shells and 3 valence electrons. The configuration is 1s² 2s² 2p⁶ 3s² 3p¹.
- Period 3 and group 2 is magnesium. It has two valence electrons in the 3s subshell.
Periodic trends
Atomic radius is half the distance between two identical bonded atoms. Three factors affect it: number of shells, effective nuclear charge and shielding.
- It decreases across a period, because the nuclear charge pulls the electron cloud closer.
- It increases down a group, because extra shells add shielding.
- A positive ion is generally smaller than its atom. A negative ion is generally larger.
Ionization energy. The first ionization energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms. The book gives these values:
- Na: i₁ = 494 kJ mol⁻¹.
- Ca: i₁ = 590, i₂ = 1150 and i₃ = 4940 kJ mol⁻¹.
- Each next electron costs more than the one before.
It increases across a period and decreases down a group. In group 1: Li > Na > K > Rb > Cs. Noble gases have the highest values. Oxygen (1365) is lower than nitrogen (1403) because nitrogen has a half-filled subshell.
Electron affinity. Cl(g) + e⁻ → Cl⁻(g) releases 348.8 kJ mol⁻¹, so the sign is negative. It generally decreases down a group. The book lists halogens as At < I < Br < F < Cl. Fluorine is smaller than chlorine yet has the lower value.
Electronegativity is the power of an atom to attract a shared pair of electrons. Pauling's scale has no unit. Alkali metals have the lowest values (0.8) and fluorine the highest (4.0). It increases across a period and decreases down a group. In the halogens it falls from F (4.0) to I (2.5).
Key points: period 3 chemistry
Sodium and magnesium.
- With water: sodium reacts vigorously. Magnesium reacts more slowly, but faster with steam. 2Na + 2H₂O → 2NaOH + H₂ Mg + 2H₂O(g) → MgO + 2H₂
- With oxygen: sodium burns with a golden yellow flame. Magnesium burns with an intense white flame to give MgO. Sodium is kept under kerosene oil.
- With chlorine: 2Na + Cl₂ → 2NaCl and Mg + Cl₂ → MgCl₂.
Oxides. Basic oxides come from metals (Na₂O, CaO). Acidic oxides come from non-metals (SO₂, SO₃, P₂O₅). Amphoteric oxides such as Al₂O₃ react with both acids and bases.
Chlorides. NaCl is neutral (pH = 7) and MgCl₂ gives pH = 6.5. From aluminium to sulfur the chlorides react with water to give acidic solutions. This is hydrolysis. AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl.
Oxidation numbers (book Table 1.2).
| Oxide | Number | Chloride | Number |
|---|---|---|---|
| Na₂O | +1 | NaCl | +1 |
| MgO | +2 | MgCl₂ | +2 |
| Al₂O₃ | +3 | AlCl₃ | +3 |
| SiO₂ | +4 | SiCl₄ | +4 |
| SO₂ / SO₃ | +4 / +6 | PCl₅ / PCl₃ | +5 / +3 |
Common mistakes in the exam
- Mixing the years. Mendeleev is 1869 and Moseley is 1913.
- Saying Mendeleev sorted by atomic number. He used atomic mass.
- Saying atoms get bigger across a period. They get smaller.
- Saying a negative ion is smaller than its atom. It is larger.
- Forgetting the sign of electron affinity. If energy is released, the sign is negative.
- Calling Al₂O₃ basic. It is amphoteric.
Practice questions
- Which scientist arranged 63 elements by atomic mass and left gaps? Answer: Mendeleev, in 1869.
- What did Moseley find in 1913? Answer: The exact atomic numbers, using X-ray emission.
- An element is in period 3 and group 13. Write its configuration. Answer: 1s² 2s² 2p⁶ 3s² 3p¹.
- Why is the ionization energy of nitrogen higher than oxygen? Answer: Nitrogen has a half-filled 2p subshell, which is stable. Values: 1403 and 1365 kJ mol⁻¹.
- Write the reaction of sodium with water. Answer: 2Na + 2H₂O → 2NaOH + H₂.
- What is the oxidation number of sulfur in SO₂ and SO₃? Answer: +4 in SO₂ and +6 in SO₃.
- Why is AlCl₃ an acidic chloride but NaCl is neutral? Answer: AlCl₃ reacts with water by hydrolysis and gives an acidic solution. NaCl does not react with water, so its solution has pH = 7.
Quick revision
- Mendeleev 1869: 63 elements, atomic mass, gaps. Moseley 1913: atomic number.
- 7 periods, 18 groups. Period number is shells. Group gives valence electrons.
- Across a period: radius down, ionization energy up, electronegativity up.
- Oxidation number in period 3 oxides: +1 to +6.
Go deeper in the lessons History of the Periodic Table (/learn/fsc-1/chemistry/history-of-the-periodic-table), The Modern Periodic Table: Groups, Blocks and Families (/learn/fsc-1/chemistry/modern-periodic-table), Periodic Trends (/learn/fsc-1/chemistry/periodic-trends), Oxides (/learn/fsc-1/chemistry/oxides), Chlorides and Other Halides (/learn/fsc-1/chemistry/halides) and Oxidation Numbers and Hydration Energy (/learn/fsc-1/chemistry/oxidation-state-and-hydration-energy).
Ready to practise this? Continue in 1st Year Chemistry.